Here’s my problem:
- iMac late 2012 model
- internal HDD dead
- been running El Capitan on external SSD for several years
- upgraded to Sierra without issue
- cannot upgrade to High Sierra, Mojave or Catalina. Get message from installer that "You may not install this volume because the computer is missing a firmware partition.”
- anybody encounter and solve similar issue? Seems from other forums that it is related to recovery disk partition on OEM internal HDD (which is completely unusable in my case).
Advice welcome.